Ca²âº signals promote GLUT4 exocytosis and reduce its endocytosis in muscle cells.

Elevating cytosolic Ca(2+) stimulates glucose uptake in skeletal muscle, but how Ca(2+) affects intracellular traffic of GLUT4 is unknown. In tissue, changes in Ca(2+) leading to contraction preclude analysis of the impact of individual, Ca(2+)-derived signals. In L6 muscle cells stably expressing GLUT4myc, the Ca(2+) ionophore ionomycin raised cytosolic Ca(2+) and caused a gain in cell surface GLUT4myc. Extra- and intracellular Ca(2+) chelators (EGTA, BAPTA-AM) reversed this response. Ionomycin activated calcium calmodulin kinase II (CaMKII), AMPK, and PKCs, but not Akt. Silencing CaMKIIδ or AMPKα1/α2 partly reduced the ionomycin-induced gain in surface GLUT4myc, as did peptidic or small molecule inhibitors of CaMKII (CN21) and AMPK (Compound C). Compared with the conventional isoenzyme PKC inhibitor Gö6976, the conventional plus novel PKC inhibitor Gö6983 lowered the ionomycin-induced gain in cell surface GLUT4myc. Ionomycin stimulated GLUT4myc exocytosis and inhibited its endocytosis in live cells. siRNA-mediated knockdown of CaMKIIδ or AMPKα1/α2 partly reversed ionomycin-induced GLUT4myc exocytosis but did not prevent its reduced endocytosis. Compared with Gö6976, Gö6983 markedly reversed the slowing of GLUT4myc endocytosis triggered by ionomycin. In summary, rapid Ca(2+) influx into muscle cells accelerates GLUT4myc exocytosis while slowing GLUT4myc endocytosis. CaMKIIδ and AMPK stimulate GLUT4myc exocytosis, whereas novel PKCs reduce endocytosis. These results identify how Ca(2+)-activated signals selectively regulate GLUT4 exocytosis and endocytosis in muscle cells.

Regulation of glucose transporter 4 traffic by energy deprivation from mitochondrial compromise.

Skeletal muscle is the major store and consumer of fatty acids and glucose. Glucose enters muscle through glucose transporter 4 (GLUT4). Upon insufficient oxygen availability or energy compromise, aerobic metabolism of glucose and fatty aids cannot proceed, and muscle cells rely on anaerobic metabolism of glucose to restore cellular energy status. An increase in glucose uptake into muscle is a key response to stimuli requiring rapid energy supply. This chapter analyses the mechanisms of the adaptive regulation of glucose transport that rescue muscle cells from mitochondrial uncoupling. Under these conditions, the initial drop in ATP recovers rapidly, through a compensatory increase in glucose uptake. This adaptive response involves AMPK activation by the initial ATP drop, which elevates cell surface GLUT4 and glucose uptake. The gain in surface GLUT4 involves different signals and routes of intracellular traffic compared with those engaged by insulin. The hormone increases GLUT4 exocytosis through phosphatidylinositol 3-kinase and Akt, whereas energy stress retards GLUT4 endocytosis through AMPK and calcium inputs. Given that energy stress is a component of muscle contraction, and that contraction activates AMPK and raises cytosolic calcium, we hypothesize that the increase in glucose uptake during contraction may also involve a reduction in GLUT4 endocytosis.

Reduction of insulin-stimulated glucose uptake in L6 myotubes by the protein kinase inhibitor SB203580 is independent of p38MAPK activity.

Insulin increases glucose uptake through translocation of the glucose transporter GLUT4 to the plasma membrane. We previously showed that insulin activates p38MAPK, and inhibitors of p38MAPKalpha and p38MAPKbeta (e.g. SB203580) reduce insulin-stimulated glucose uptake without affecting GLUT4 translocation. This observation suggested that insulin may increase GLUT4 activity via p38alpha and/or p38beta. Here we further explore the possible participation of p38MAPK through a combination of molecular strategies. SB203580 reduced insulin stimulation of glucose uptake in L6 myotubes overexpressing an SB203580-resistant p38alpha (drug-resistant p38alpha) but barely affected phosphorylation of the p38 substrate MAPK-activated protein kinase-2. Expression of dominant-negative p38alpha or p38beta reduced p38MAPK phosphorylation by 70% but had no effect on insulin-stimulated glucose uptake. Gene silencing via isoform-specific small interfering RNAs reduced expression of p38alpha or p38beta by 60-70% without diminishing insulin-stimulated glucose uptake. SB203580 reduced photoaffinity labeling of GLUT4 by bio-LC-ATB-BMPA only in the insulin-stimulated state. Unless low levels of p38MAPK suffice to regulate glucose uptake, these results suggest that the inhibition of insulin-stimulated glucose transport by SB203580 is likely not mediated by p38MAPK. Instead, changes experienced by insulin-stimulated GLUT4 make it susceptible to inhibition by SB203580.

Troglitazone causes acute mitochondrial membrane depolarisation and an AMPK-mediated increase in glucose phosphorylation in muscle cells.

AIMS/HYPOTHESIS: Troglitazone was the first thiazolidinedione (TZD) approved for clinical use, exerting hypoglycaemic effects related to its action as a ligand of the peroxisome proliferator-activated receptor gamma receptor in adipocytes. However, emerging evidence suggests that mitochondrial function may be affected by troglitazone, and that skeletal muscle cells acutely respond to troglitazone by enhancing glucose uptake. The aim of the present study was to determine the cellular mechanisms by which troglitazone acutely stimulates glucose utilisation in skeletal muscle cells. METHODS: L6 cells overexpressing GLUT4myc were incubated with troglitazone. Glucose uptake, transport and phosphorylation as well as AMP-activated protein kinase (AMPK) signalling and insulin signalling were examined. Changes in mitochondrial membrane potential were measured using the J-aggregate-forming dye JC-1. AMPK signalling was interfered with using AMPK alpha1/alpha2 siRNA. RESULTS: Troglitazone acutely (in 10 min) reduced the mitochondrial membrane potential in L6GLUT4myc myotubes and robustly stimulated AMPK activity. Following 30 min of incubation with troglitazone or insulin, 2-deoxyglucose uptake was stimulated 1.5- and 2.1-fold respectively, and in cells treated with troglitazone, a 1.8-fold increase in the 2-deoxyglucose-6-phosphate:2-deoxyglucose ratio was observed. Moreover, contrary to insulin, troglitazone did not significantly stimulate 3-O-methylglucose uptake. Unlike insulin, troglitazone did not increase surface GLUT4myc content and did not increase IRS1-associated phosphatidylinositol 3-kinase activity or Akt phosphorylation on T308 and S473. Interestingly, interfering with troglitazone-induced activation of AMPK by decreasing the expression of the enzyme using siRNA inhibited the stimulation of 2-deoxyglucose uptake by the TZD. CONCLUSIONS/INTERPRETATION: We propose that troglitazone acutely increases glucose flux in muscle via an AMPK-mediated increase in glucose phosphorylation.

Indinavir uncovers different contributions of GLUT4 and GLUT1 towards glucose uptake in muscle and fat cells and tissues.

AIMS/HYPOTHESIS: Insulin-dependent glucose influx in skeletal muscle and adipocytes is believed to rely largely on GLUT4, but this has not been confirmed directly. We assessed the relative functional contribution of GLUT4 in experimental models of skeletal muscle and adipocytes using the HIV-1 protease inhibitor indinavir. METHODS: Indinavir (up to 100 micro mol/l) was added to the glucose transport solution after insulin stimulation of wild-type L6 muscle cells, L6 cells over-expressing either GLUT4myc or GLUT1myc, 3T3-L1 adipocytes, isolated mouse brown or white adipocytes, and isolated mouse muscle preparations. RESULTS: 100 micro mol/l indinavir inhibited 80% of both basal and insulin-stimulated 2-deoxyglucose uptake in L6GLUT4myc myotubes and myoblasts, but only 25% in L6GLUT1myc cells. Cell-surface density of glucose transporters was not affected. In isolated soleus and extensor digitorum longus muscles, primary white and brown adipocytes, insulin-stimulated glucose uptake was inhibited 70 to 80% by indinavir. The effect of indinavir on glucose uptake was variable in 3T3-L1 adipocytes, averaging 45% and 67% inhibition of basal and maximally insulin-stimulated glucose uptake, respectively. In this cell, fractional inhibition of glucose uptake by indinavir correlated positively with the fold-stimulation of glucose uptake by insulin, and was higher with sub-maximal insulin concentrations. The latter finding coincided with an increase only in GLUT4, but not GLUT1, in plasma membrane lawns. CONCLUSION/INTERPRETATION: Indinavir is a useful tool to assess different functional contributions of GLUT4 to glucose uptake in common models of skeletal muscle and adipocytes.

VAMP2, but not VAMP3/cellubrevin, mediates insulin-dependent incorporation of GLUT4 into the plasma membrane of L6 myoblasts.

Like neuronal synaptic vesicles, intracellular GLUT4-containing vesicles must dock and fuse with the plasma membrane, thereby facilitating insulin-regulated glucose uptake into muscle and fat cells. GLUT4 colocalizes in part with the vesicle SNAREs VAMP2 and VAMP3. In this study, we used a single-cell fluorescence-based assay to compare the functional involvement of VAMP2 and VAMP3 in GLUT4 translocation. Transient transfection of proteolytically active tetanus toxin light chain cleaved both VAMP2 and VAMP3 proteins in L6 myoblasts stably expressing exofacially myc-tagged GLUT4 protein and inhibited insulin-stimulated GLUT4 translocation. Tetanus toxin also caused accumulation of the remaining C-terminal VAMP2 and VAMP3 portions in Golgi elements. This behavior was exclusive to these proteins, because the localization of intracellular myc-tagged GLUT4 protein was not affected by the toxin. Upon cotransfection of tetanus toxin with individual vesicle SNARE constructs, only toxin-resistant VAMP2 rescued the inhibition of insulin-dependent GLUT4 translocation by tetanus toxin. Moreover, insulin caused a cortical actin filament reorganization in which GLUT4 and VAMP2, but not VAMP3, were clustered. We propose that VAMP2 is a resident protein of the insulin-sensitive GLUT4 compartment and that the integrity of this protein is required for GLUT4 vesicle incorporation into the cell surface in response to insulin.

Role of the actin cytoskeleton in insulin action.

Insulin has diverse effects on cells, including stimulation of glucose transport, gene expression, and alterations of cell morphology. The hormone mediates these effects by activation of signaling pathways which utilize, 1) adaptor molecules such as the insulin receptor substrates (IRS), the Src and collagen homologs (Shc), and the growth factor receptor binding protein 2 (Grb2); 2) lipid kinases such as phosphatidylinositol 3-kinase (PI 3-Kinase); 3) small G proteins; and 4) serine, threonine, and tyrosine kinases. The activation of such signaling molecules by insulin is now well established, but we do not yet fully understand the mechanisms integrating these seemingly diverse pathways. Here, we discuss the involvement of the actin cytoskeleton in the propagation and regulation of insulin signals. In muscle cells in culture, insulin induces a rapid actin filament reorganization that coincides with plasma membrane ruffling and intense accumulation of pinocytotic vesicles. Initiation of these effects of insulin requires an intact actin cytoskeleton and activation of PI 3-kinase. We observed recruitment PI 3-kinase subunits and glucose transporter proteins to regions of reorganized actin. In both muscle and adipose cells, actin disassembly inhibited early insulin-induced events such as recruitment of glucose transporters to the cell surface and enhanced glucose transport. Additionally, actin disassembly inhibited more prolonged effects of insulin, including DNA synthesis and expression of immediate early genes such as c-fos. Intact actin filaments appear to be essential for mediation of early events such as association of Shc with Grb2 in response to insulin, which leads to stimulation of gene expression. Preliminary observations support a role for focal adhesion signaling complexes in insulin action. These observations suggest that the actin cytoskeleton facilitates propagation of the morphological, metabolic, and nuclear effects of insulin by regulating proper subcellular distribution of signaling molecules that participate in the insulin signaling pathway.

Protein kinase B/Akt participates in GLUT4 translocation by insulin in L6 myoblasts.

L6 myoblasts stably transfected with a GLUT4 cDNA harboring an exofacial myc epitope tag (L6-GLUT4myc myoblasts) were used to study the role of protein kinase B alpha (PKBalpha)/Akt1 in the insulin-induced translocation of GLUT4 to the cell surface. Surface GLUT4myc was detected by immunofluorescent labeling of the myc epitope in nonpermeabilized cells. Insulin induced a marked translocation of GLUT4myc to the plasma membrane within 20 min. This was prevented by transient transfection of a dominant inhibitory construct of phosphatidylinositol (PI) 3-kinase (Deltap85alpha). Transiently transfected cells were identified by cotransfection of green fluorescent protein. A constitutively active PKBalpha, created by fusion of a viral Gag protein at its N terminus (GagPKB), increased the cell surface density of GLUT4myc compared to that of neighboring nontransfected cells. A kinase-inactive, phosphorylation-deficient PKBalpha/Akt1 construct with the mutations K179A (substitution of alanine for the lysine at position 179), T308A, and S473A (AAA-PKB) behaved as a dominant-negative inhibitor of insulin-dependent activation of cotransfected wild-type hemagglutinin (HA)-tagged PKB. Furthermore, AAA-PKB markedly inhibited the insulin-induced phosphorylation of cotransfected BAD, demonstrating inhibition of the endogenous PKB/Akt. Under the same conditions, AAA-PKB almost entirely blocked the insulin-dependent increase in surface GLUT4myc. PKBalpha with alanine substitutions T308A and S473A (AA-PKB) or K179A (A-PKB) alone was a less potent inhibitor of insulin-dependent activation of wild-type HA-PKB or GLUT4myc translocation than was AAA-PKB. Cotransfection of AAA-PKB with a fourfold DNA excess of HA-PKB rescued insulin-stimulated GLUT4myc translocation. AAA-PKB did not prevent actin bundling (membrane ruffling), though this response was PI 3-kinase dependent. Therefore, it is unlikely that AAA-PKB acted by inhibiting PI 3-kinase signaling. These results outline an important role for PKBalpha/Akt1 in the stimulation of glucose transport by insulin in muscle cells in culture.

Opposite effects of insulin on focal adhesion proteins in 3T3-L1 adipocytes and in cells overexpressing the insulin receptor.

Insulin can regulate the abundance and organization of filamentous actin within cells in culture. Early studies using cell lines that overexpress the insulin receptor demonstrated that insulin caused a rapid reversible disassembly of actin filaments that coincided with the rapid tyrosine dephosphorylation of focal adhesion kinase. We have extended these studies by demonstrating that paxillin, another focal adhesion protein, and Src undergo tyrosine dephosphorylation in response to insulin in Chinese hamster ovary (CHO) and rat hepatoma (HTC) cells that overexpress the insulin receptor. This contrasted with the effect of insulin in parental CHO and HTC cells in which focal adhesion proteins were not dephosphorylated in response to the hormone. In addition, insulin caused a dispersion of focal adhesion proteins and disruption of actin filament bundles only in cells that overexpressed the insulin receptor. Moreover, in 3T3-L1 adipocytes, which are considered prototypic insulin-responsive cells, actin filament assembly was stimulated, and focal adhesion protein tyrosine phosphorylation was not altered. 3T3-L1 cells have more insulin receptors than either parental CHO or HTC cells but have fivefold less insulin receptors than the overexpressing cell lines. We hypothesize that a threshold may exist in which the overexpression of insulin receptors determines how insulin signaling pathways regulate the actin cytoskeleton.

The ras-related protein rad associates with the cytoskeleton in a non-lipid-dependent manner.

Rad is the prototypic member of a new family of Ras-related proteins (Rad, Gem, and Kir) which lack typical C-terminal amino acid motifs for isoprenylation. In mouse C2C12 muscle cell lines about 50% of Rad protein resides in the cytosol and behaves as a hydrophilic protein partitioning away from TX-114. The remainder of Rad is associated with plasma and internal membranes. The association of Rad with the membrane does not occur through the lipid bilayer, but instead depends on the interaction of Rad with the cytoskeleton or membrane skeleton. In contrast to Ras, biosynthetic labeling of cellular proteins in C2Cl2 cells with [3H]palmitic acid demonstrates that Rad is not modified with this fatty acid, and inhibition of isoprenylation with lovastatin treatment has no effect on Rad subcellular distribution. Furthermore, removal of the C-terminal 11 amino acids that are precisely conserved in all three Rad family members has no effect on Rad subcellular distribution. Addition of the 9 amino acids from the C-terminus of H-Ras to the truncated Rad protein results in a redistribution of Rad from the cytosol to the membrane skeleton without the presence of any detectable lipid modification of the chimeric protein. These data suggest that Rad possesses unique cellular localization signals which, in contrast to other Ras-related family members, do not depend on the lipid modification of the C-terminus.

Actin filaments participate in the relocalization of phosphatidylinositol3-kinase to glucose transporter-containing compartments and in the stimulation of glucose uptake in 3T3-L1 adipocytes.

Insulin stimulates the rate of glucose uptake into muscle and adipose cells by translocation of glucose transporters from an intracellular storage pool to the plasma membrane. This event requires the prior activation of phosphatidylinositol 3-kinase (PI 3-kinase). Here we report that insulin causes an increase in wortmannin-sensitive PI 3-kinase activity and a gain in the enzyme's regulatory and catalytic subunits p85alpha and p110beta (but not p110alpha) in the intracellular compartments containing glucose transporters. The hormone also caused a marked reorganization of actin filaments, which was prevented by cytochalasin D. Cytochalasin D also decreased significantly the insulin-dependent association of PI 3-kinase activity and the levels of insulin receptor substrate (IRS)-1, p85alpha and p110beta with immunopurified GLUT4-containing compartments. In contrast, the drug did not alter the insulin-induced tyrosine phosphorylation of IRS-1, the association of PI 3-kinase with IRS-1, or the stimulation of PI 3-kinase by insulin in anti-(IRS-1) or anti-p85 immunoprecipitates from whole cell lysates. Cytochalasin D, and the chemically unrelated latrunculin B, which also inhibits actin filament reassembly, prevented the insulin stimulation of glucose transport by approx. 50%. Cytochalasin D decreased by about one-half the insulin-dependent translocation to the plasma membrane of the GLUT1 and GLUT4 glucose transporters. The results suggest that the existence of intact actin filament is correlated with the full recruitment of glucose transporters by insulin. The underlying function of the actin filaments might be to facilitate the insulin-mediated association of the p85-p110 PI 3-kinase with glucose-transporter-containing compartments.

Rad and Rad-related GTPases interact with calmodulin and calmodulin-dependent protein kinase II.

Members of the Rad family of GTPases (including Rad, Gem, and Kir) possess several unique features of unknown function in comparison to other Ras-like proteins, with major N-terminal and C-terminal extensions, a lack of typical prenylation motifs, and several non-conservative changes in the sequence of the GTP binding domain. Here we show that Rad and Gem bind to calmodulin (CaM)-Sepharose in vitro in a calcium-dependent manner and that Rad can be co-immunoprecipitated with CaM in C2C12 cells. The interaction is influenced by the guanine nucleotide binding state of Rad with the GDP-bound form exhibiting 5-fold better binding to CaM than the GTP-bound protein. In addition, the dominant negative mutant of Rad (S105N) which binds GDP, but not GTP, exhibits enhanced binding to CaM in vivo when expressed in C2C12 cells. Peptide competition studies and expression of deletion mutants of Rad localize the binding site for CaM to residues 278-297 at the C terminus of Rad. This domain contains a motif characteristic of a calmodulin-binding region, consisting of numerous basic and hydrophobic residues. In addition, we have identified a second potential regulatory domain in the extended N terminus of Rad which, when removed, decreases Rad protein expression but increases the binding of Rad to CaM. The ability of Rad mutants to bind CaM correlates with their localization in cytoskeletal fractions of C2C12 cells. Immunoprecipitates of calmodulin-dependent protein kinase II, the cellular effector of Ca2+-calmodulin, also contain Rad, and in vitro both Rad and Gem can serve as substrates for this kinase. Thus, the Rad family of GTP-binding proteins possess unique characteristics of binding CaM and calmodulin-dependent protein kinase II, suggesting a role for Rad-like GTPases in calcium activation of serine/threonine kinase cascades.

Overexpression of Rad inhibits glucose uptake in cultured muscle and fat cells.

Rad is a Ras-like GTPase that was isolated by subtraction cloning of human muscle and shown to have increased expression in some individuals with Type II diabetes. To ascertain the potential role of Rad in insulin-mediated signaling, we have overexpressed Rad in myocyte and adipocyte cell lines. Expression of Rad resulted in a 50-90% reduction in insulin-stimulated 2-deoxyglucose glucose uptake in C2C12 murine myotubes, L6 rat myotubes, and 3T3-L1 adipocytes and a 25% reduction in 3-O-methylglucose uptake in 3T3-L1 adipocytes. This occurred despite unaltered levels of glucose transporter expression, with no detectable change in Glut4 translocation and with no alteration in insulin receptor or substrate phosphorylation or phosphatidylinositol 3-kinase activity. These data indicate that Rad is a negative regulator of glucose uptake and that this effect may be due to a decrease in the intrinsic activity of the transporter molecules, rather than an effect on the translocation of Glut4.

Rad, a novel Ras-related GTPase, interacts with skeletal muscle beta-tropomyosin.

Rad, a prototypic member of a subfamily of Ras-related GTPases, is overexpressed in skeletal muscle of type II diabetic humans. By expression screening of mouse embryo and human skeletal muscle cDNA libraries, we found that Rad interacted with skeletal muscle beta-tropomyosin. In the mouse skeletal muscle cell line C2C12, this interaction was significantly increased by the calcium ionophore A23187. A23187 also caused a time- and concentration-dependent decrease in total cellular Rad with increased interaction between tropomyosin and Rad in the detergent-soluble fraction and the appearance of Rad in the cytoskeleton. In C2C12 cells stably overexpressing a putative dominant negative mutant of Rad (S105N), there was an increase in the amount of tropomyosin in Rad immunoprecipitates. In cells overexpressing wild type Rad, much of Rad was associated with the cytoskeleton and was no longer responsive to A23187. In far-Western blotting and guanine nucleotide saturation studies, GDP-Rad bound to tropomyosin far better than GTP-Rad. We conclude that Rad interacts with skeletal muscle beta-tropomyosin and the cytoskeleton in a guanine nucleotide-dependent manner. These data suggest that Rad may be involved in skeletal muscle motor function and cytoskeletal organization.

Unusual insulin binding to cells expressing an insulin receptor mutated at cysteine 524.

The involvement of cysteine 524 of the insulin receptor in an intermolecular (class I) disulfide bond between the two alpha-subunits was investigated using site-directed mutagenesis. The oligomeric structure of the mutated receptor was partially disrupted, although a significant portion of the receptor remained in its heterotetrameric form. Interestingly, the mutated insulin receptor heterotetramer was more susceptible than the wildtype receptor to reduction to heterodimers by low concentrations of dithiothreitol. Insulin binding to solubilized mutant receptors was normal and the mutant receptors had normal affinity for insulin, but insulin binding to cells expressing mutant insulin receptors displayed positive cooperativity. Cysteine 524 is most likely involved in a class I disulfide bond and receptors mutated at this site displayed unusual insulin binding properties only in the cellular environment.

Structural disruption of the trans-Golgi network does not interfere with the acute stimulation of glucose and amino acid uptake by insulin-like growth factor I in muscle cells.

The effects of insulin-like growth factor I (IGF-I) on glucose and amino acid uptake were investigated in fully differentiated L6 muscle cells, in order to determine whether the two processes are functionally related. Transport of both glucose and amino acid (methylaminoisobutyric acid, MeAIB) was activated rapidly in response to IGF-I. Stimulation reached a peak within 30 min and was sustained for up to 90 min. Maximal activation of either glucose or MeAIB transport was achieved at 3 nM IGF-I; the half-maximal activation (ED50) of glucose transport was at 107 pM and that of MeAIB transport was at 36 pM. Stimulation of amino acid uptake occurred in the absence or presence of glucose, suggesting that this response is not secondary to increased glucose intake. Incubation of cells for 1 h with Brefeldin A (5 micrograms/ml), which disassembles the Golgi apparatus and inhibits the secretory pathway in eukaryotic cells, had no effect on the acute IGF-I activation of glucose and MeAIB transport. Moreover, Brefeldin A caused wide redistribution of the trans-Golgi antigen TGN38, as assessed by subcellular fractionation, without affecting the distribution of glucose transporters. The finding that the degree of activation, time response and sensitivity to IGF-I and Brefeldin A were similar for both glucose and MeAIB transport suggests commonalities in the IGF-I mechanism of recruitment of glucose transporters and stimulation of amino acid transport through System A. An integral trans-Golgi network does not appear to be required for the acute IGF-I stimulation of glucose or amino acid transport, even though stimulation of glucose transport occurs through recruitment of glucose transporters from intracellular stores in these cells. We propose that the donor site of glucose transporters (and perhaps of amino acid transporters) involved in the acute response to IGF-I lies beyond the trans-Golgi network, perhaps in an endosomal compartment in close proximity to the plasma membrane.

Detection of the GLUT3 facilitative glucose transporter in rat L6 muscle cells: regulation by cellular differentiation, insulin and insulin-like growth factor-I.

The GLUT3 facilitative glucose transporter protein was found to be expressed in rat L6 muscle cells. It was detected at both the myoblast and myotube stage. GLUT3 protein content per mg of total membrane protein increased significantly during L6 cell differentiation. Subcellular fractionation demonstrated that the GLUT3 protein was predominantly localized in plasma membrane-enriched fractions of either myoblasts or myotubes. Short-term exposure of L6 myotubes to IGF-I or insulin caused a redistribution of GLUT3 protein from an intracellular membrane fraction to the plasma membrane, without affecting total membrane GLUT3 protein content. Long-term exposure of L6 myotubes to IGF-I produced an increase of GLUT3 protein in total membranes and all subcellular membrane fractions, especially the plasma membrane. We propose that the GLUT3 glucose transporter may play an important role in glucose metabolism in developing muscle.

Stimulation of hexose transport by metformin in L6 muscle cells in culture.

L6 muscle cells grown in culture to the stage of fused myotubes were incubated with the oral hypoglycemic drug metformin to test the effects of this drug on glucose transport. Metformin increased the initial rate of uptake of 2-deoxyglucose and 3-O-methylglucose. The effect was time dependent, with half-maximal stimulation at 5-6 h and maximal stimulation by about 16 h. The stimulation of hexose uptake was not prevented by cycloheximide. In 15 mM glucose medium, the basal rate of transport was lower than in 5 mM glucose medium. The stimulation of hexose uptake by metformin was comparable in absolute units in both media; hence, relative to basal uptake, stimulation was greater in the high glucose medium than in the low glucose medium. In 5 mM glucose medium, half-maximal stimulation was obtained with 800 microM metformin when tested for 24 h. The stimulation of hexose transport by metformin was only detectable in fused myotubes and not in perfusion myoblasts. No significant changes were observed in glucose transporter levels in total cell membranes from L6 myotubes (measured as D-glucose-protectable binding sites for cytochalasin-B) or in the total levels of the immunoreactive glucose transporter isoforms GLUT4 or GLUT1. It is concluded that metformin stimulates hexose transport into differentiated muscle cells by acting at a posttranslational level. We speculate that this might also constitute the basis for the ability of the drug to lower glycemia in diabetic individuals.

Insulin-stimulated glucose transport in circulating mononuclear cells from nondiabetic and IDDM subjects.

The objectives of this study were 1) to evaluate glucose transport and its regulation by insulin in easily accessible human cells, 2) to investigate the glucose transporter isoforms involved, and 3) to establish whether a defect in glucose transport is associated with peripheral insulin resistance, which is common in insulin-dependent diabetes mellitus (IDDM) patients. We measured 2-deoxyglucose (2-DG) uptake in circulating mononuclear cells from 23 nondiabetic adults, 16 adults with IDDM, and 10 children with IDDM. Circulating mononuclear cells were separated from whole blood by Ficoll gradients and incubated with +/- 1 nM insulin. 2-DG uptake was measured after incubation with [3H]2-DG and cell separation through corn oil-phthalate. Cytochalasin B-inhibitable 2-DG uptake (basal and insulin stimulated) was higher in control than in IDDM subjects (P less than 0.001). Insulin significantly increased 2-DG uptake or 3-O-methylglucose uptake in both groups. Basal and insulin-stimulated 2-DG uptake was similar for adults and children with IDDM and did not correlate with age or body mass index in any group or disease duration, insulin dosage, or HbA1c in IDDM. In separated monocytes and lymphocytes, 2-DG uptake increased in response to insulin only in the monocyte population. Insulin dose-response curves indicated maximal stimulation of hexose uptake at 1-2 nM insulin for both control and diabetic subjects and demonstrated a significant decrease in maximal insulin response in the latter. Immunoblotting with specific antibodies revealed that circulating mononuclear cells and separated monocytes express the GLUT1 but not the GLUT4 isoform of the glucose transporter.(ABSTRACT TRUNCATED AT 250 WORDS)